In arrangements of this kind, it is customary to slide the filter band horizontally according to the art of the extrusion press by means of heat-softened thermoplastic in order to replace, in each case, a section of the filter permeated by impurities with a part that is still clean.
However, in arrangements with fishtail dies for manufacturing foil, by which the die slot has by comparison a very large width and, hence, the active filter band has a disproportionately large length, this art leads to an increased clogging-up of the filter on the exit side of the filter band, clogging occurs because the filter band can be moved only very slowly at right angles the direction of flow of the warm, soft thermoplastic, and, consequently, there is practically never the same rate of passage at all places in front of the die slot. Moreover, there also results an uneven passage of the soft, warm mass of plastic through the filter, i.e. consecutively through the fishtail die, resulting in an uneven foil.